1. Field of the Invention
The present invention relates to a lockup force control apparatus for a fluid coupling in a vehicle with an automatic transmission.
2. Description of Related Art
In general, an automatic transmission equipped in an automobile is built by combining a torque converter as a kind of fluid coupling, and a transmission gear mechanism, and switches the power transmission route by selectively controlling lockup and release of a plurality of friction elements such as clutches, brakes, and the like that constitute the transmission gear mechanism, thereby automatically changing gears to a desired shift range. In recent years, the torque converter comprises a lockup clutch that directly couples an input element from an engine, and an output element to the transmission gear mechanism.
More specifically, the torque converter comprises a pump (input element) which is coupled to the engine output shaft and rotates together with the engine output shaft, a turbine (output element) disposed to oppose the pump, and a stator interposed between the pump and turbine to increase torque. The torque converter transmits the rotation of the pump to the turbine via hydraulic oil, and outputs the transmitted rotation to the transmission gear mechanism via a turbine shaft coupled to the turbine. In addition to these components, a lockup clutch as a friction element is interposed between the pump and turbine. Upon locking up the lockup clutch, the pump and engine output shaft as the input side of the torque converter are directly coupled to the turbine and turbine shaft as the output shaft, and they can be rotated together.
An automobile equipped with the lockup clutch pre-stores a characteristic map pertaining to the lockup state of the lockup clutch set up with traveling conditions such as the degree of throttle opening, vehicle velocity, and the like as parameters. By substituting the current traveling conditions of the automobile into this characteristic map, the lockup state of the lockup clutch suitable for the current traveling conditions is determined.
For example, when the traveling conditions of the automobile fall within a high-load/low-vehicle velocity range (converter range), since the torque-up effect of the torque converter, the shock absorption effect during transmission, and the like are required, the lockup clutch is fully released, and is set in a converter state (release state). On the other hand, when the traveling conditions fall within a low-load/high-vehicle velocity range (lockup range) that does not so require severe transmission shock absorption effect or the like, the lockup clutch is fully locked up and is set in a lockup state (full lockup state) so as to enhance the power transmission efficiency of the torque converter and to improve the mileage or the like of an engine.
When the traveling conditions of the automobile fall within a low-load/low-vehicle velocity range (slip range), the lockup clutch is set in a slip state so as to balance between the torque-up effect, shock absorption effect, and the like obtained in the converter state and the mileage and the like obtained in the lockup state.
For example, Japanese Patent Laid-Open No. 61-99763 discloses slip control for converging the slip amount of the lockup clutch to a predetermined target slip amount in the slip range. In this slip control, the actual slip amount of the lockup clutch is calculated on the basis of the difference between the engine rotational speed (that of the engine output shaft) and the turbine rotational speed (that of the turbine shaft), and the lockup force of the lockup clutch is feedback-controlled so that the actual slip amount converges to the target slip amount which is set in advance or calculated in correspondence with the traveling conditions at that time.
During the slip control, the lockup clutch is set in the slip state, and the slip amount is controlled to converge to the predetermined target slip amount. Upon examining the torque converter state at that time in terms of the torque-up effect, for example, when the driver has pressed the accelerator pedal and the engine rotational speed has risen, since the actual slip amount deviates in a direction to become larger than the target slip amount, the torque converter state relatively approaches the converter state, and the torque-up effect becomes larger as compared to the case wherein the actual slip amount has converged to the target slip amount. On the other hand, when the actual slip amount has converged to the target slip amount again under the slip control, since the torque converter state relatively approaches the lockup state, the torque-up effect becomes smaller as compared to the case wherein the actual slip amount has deviated in a direction to become larger than the target slip amount. Hence, the driver feels acceleration or deceleration during such control, and recognizes the magnitude of such acceleration or deceleration as drive feeling in correspondence with accelerator pedal operation by driver oneself while the automobile is traveling on a flat road.
When the automobile has reached an upward slope after the flat road and the driver has additionally pressed the accelerator pedal to maintain the current vehicle velocity, the following problem may be posed. More specifically, the engine rotational speed rises in correspondence with the driver's accelerator pedal operation. In addition, since a drive load arising from the upward slope acts on the automobile, the turbine rotational speed drops. Hence, the actual slip amount deviates from the target slip amount beyond the driver's accelerator pedal operation, and the driver feels larger acceleration than the drive feeling that the driver recognized so far in correspondence with accelerator pedal operation by driver oneself on the flat road. Also, when the actual slip amount has converged to the target slip amount under the slip control, the driver feels larger deceleration than the expected drive feeling. When the driver has pressed the accelerator pedal deeper since the driver has felt larger deceleration than the drive feeling of the driver, the traveling conditions shift from the slip range to the converter range, as indicated by an arrow a in FIG. 4, and a larger torque-up effect is given after the shift timing. Hence, the driver feels larger acceleration again, and also feels busy, i.e., unnatural, since larger acceleration and deceleration than those the driver expected frequently repeat themselves.